In telecommunication systems, the use of in-band tones for signaling is common. For example, touch-tone.RTM. phones use a tone that is formed of two component tones to send digits in a signaling scheme called dual-tone multi-frequency (DTMF). Dual tone signaling is also used for the customer premise equipment alerting signal (CAS), subscriber alerting signal (SAS) and visual screen list editing (VSLE) tones. These systems all operate by sending a tone composed of two true tones, each at a predefined frequency. Each signaling system is governed by standards which address parameters such as minimum tone duration, absolute amplitude ranges of the component tones, relative amplitude ranges between component tones, allowable frequency variation of the component tones, and tolerable noise and interference levels.
Prior art detectors typically use band separation techniques where the input signal is split and sent along parallel processing paths. For example, in a dual tone system, one processing path detects the presence of a high tone, the other path detects the presence of a low tone, and final tone validation logic determines if the other parameters are satisfied in the presence of both tones.
The prior art detectors are typically of two designs. In the first design, the processing paths filter an input signal through band select filters, each band select filter being associated with one of the component tones and rejecting the frequencies of all other tones. The band select filter output signals are then passed through limiters, which convert the signals to square waves, then through resonators or narrow bandpass filters that detect the presence of tones at the frequencies in question, and then tone validation logic that determines that the other parameters are satisfied.
In the second design, the processing paths filter an input signal through bandpass filters, each bandpass filter being associated with one of the component tones and passing that tone frequency. The signals are then passed through comparators to determine if signals of sufficient strength are present, then through tone validation logic to determine if the other parameters are satisfied.
These prior art designs are susceptible to false tone detections caused by voice and noise signals in the non-tone frequencies of the input signal.
Accordingly, it is an object of the present invention to provide a tone detection system with improved performance in the presence of speech and noise.